One area of application of electronic countermeasures is the interception and selective interference with communications traffic. A portion or band of spectrum is monitored to ascertain which channels are carrying communications traffic, and after identifying signals on frequencies which are designated as friendly, a signal regenerator, that is a transmitter device, is activated on the frequencies of the unfriendly signals with the object of interfering with and disrupting the communications on those frequencies.
Current equipment for performing the above function predominately uses superheterodyne receivers which are swept, continuously or in steps, across the band of interest. Such equipments do not offer the capability of quickly intercepting and identifying when a new transmitter switches on nor do they possess a capability to track or follow signals which are changing in frequency.
British patent specification No. 1,046,923 (Compagnie Francaise Thomson-Houston) shows one example of a swept receiver technique in which the monitored frequency range is swept with the aid of a wobbulator whose sweep is stopped at each non-friendly signal identified and whose instantaneous frequency is then used as a source for a transmitter jamming signal on that frequency. The sweep then recommences to look for the next signal to be jammed. This procedure is inherently slow, since the sweep is interrupted for each jamming transmission. In practice the number of input signals that can be handled is very restricted as the specification itself makes clear.
British patent specifications Nos. 1,278,771 and 1,450,761 (both to Siemens) disclose alternative approaches which do not have the disadvantage above-mentioned but involve complex systems and are still only capable of relatively slow operation to cover the whole bandwidth as is necessary for transmissions that may be frequency agile. Specification No. 1,278,771 uses a complex arrangement of switchable frequency converters to divide the input band into successively examined segments each segment being finally resolved into channels by a large number (e.g. 125) of filter/detector units. Specification No. 1,450,761 divides the input bandwidth up by separately tuned filters required to be maintained in alignment across the whole frequency band in question.
Reference will be made hereinafter to the adoption of a particular transformation technique. The use of transform methods as such is known. One example is Sony's British specification No. 1,538,509 which uses a particular transform technique to reduce noise in a single input video signal. The particular transform used depends on a priori assumptions of the nature of the input signal and is not considered to have any relevance to electronic counter-measures of the kind with which the invention is concerned.